Coated cardboard and a method of producing the same

ABSTRACT

A coated food cardboard that is comprised of a cardboard layer and a coating layer at least on one side of it, and a method of producing it. Between the coating layer and the cardboard surface is formed by dispersion coating a barrier layer that is comprised of binder and slate-like pigments. The layer is formed of a dispersion that is prepared by dispersing slate-like pigment in water and mixing this with a non-hydrophobic binder. The cardboard has excellent barrier properties and it can be used, for example, in such a pigment coated cardboard, in particular fresh fibre cardboard that has good properties of mineral oil migration and fat penetration prevention.

The present invention relates to a cardboard according to the preambleof claim 1.

Such a product comprises a fibre layer that has two opposite surfaces,in which case there is a coating layer at least on its first surface.

The present invention also relates to a method, according to thepreamble of claim 13, of producing a cardboard that comprises barrierproperties, and the use according to claim 27.

Barriers are used in paper and cardboard products to prevent unwantedsubstances to penetrate through the material. Such unwanted substancescan be for example: water, water vapour, oxygen, fat, flavourings andcomponents of mineral oil. In particular, it has become important toprevent the penetration of the latter ones, because it has been foundthat from a recycled fibre cardboard that is placed directly against thefood, such as cereals, mineral oil components, that are harmful tohealth and sourced from for example printing inks, migrate into thefood. It has also been suspected that such a migration can take placeindirectly from the outer package that comprises recycled fibre, throughthe fresh fibre cardboard of the inner package, into the food.

In cardboards, barriers that are extruded or applied as dispersions aregenerally used. The barrier is applied either onto the back side or thesurface of the cardboard. Barrier application is usually carried out asan off-line operation, where the cardboard manufactured at the cardboardmill is lengthwise cut into suitable rolls, packed, transported to theprocessing plant, coated with a barrier at the processing plant, packedand sent to the customer.

With regard to the art, reference is made to publications WO 2013164646A1, U.S. Pat. No. 6,545,079 B1, EP 2777934 A1 and U.S. Pat. No.6,531,196 B1.

Known solutions are associated with problems. Barrier application as anoff-line operation significantly increases production costs of thebarrier coated cardboard, compared to cardboard which is not barriercoated, and reduces its competitiveness.

In barrier material applied as a dispersion, “pinholes”, i.e. holes thatare invisible to the eye, easily appear in the barrier layer, whichholes ruin the barrier function. These pinholes may arise, for example,due to a rough base cardboard or instability of the application method.Also, off-line coating, as a separate operation, increases the risk ofquality problems.

Application of the barrier as a separate layer onto either surface ofthe cardboard, immediately affects the other operating characteristicsof the cardboard. If, for example, a barrier is applied onto the backside of a folding boxboard, it seals the surface pore structure andprevents the penetration of dispersion or hot melt adhesives. As aresult, the production of conventional cardboard boxes requires the useof expensive special solutions, such as perforations and specialadhesives.

If a barrier is applied, for example, onto the surface of a foldingboxboard, the barrier prevents the printing inks from penetration, forexample, in offset, gravure, flexo and ink jet printing. A separatebarrier layer also increases the risk of poor toner adhesion inelectrophotographic printing methods of digital printing.

A barrier that is applied onto either side of the surface of a cardboardcan also cause blocking of the cardboard when reeling. In such a case,the cardboard layers on the machine reel adhere to each other, causingserious quality and runability problems.

When hydrophobic barrier materials are used, the cardboard recirculationback to the process is aggravated due to poor pulpability.

It is an aim of the present invention to eliminate at least some of theproblems of the prior art and to provide an entirely novel solution ofproducing cardboard having barrier properties.

The invention is based on the idea that a barrier layer is formed bydispersion coating onto the fibre layer of the cardboard, which layer iscomprised of a binder and slate-like pigments. The barrier layer isarranged between the surface and at least one coating layer.Case-specifically, there may be a surface-sizing layer or a coatinglayer between the surface and the barrier layer, respectively.Preferably, the surface-sizing agent layer or the coating layer,respectively, is formed onto the surface of the cardboard layer, priorto the application of the barrier layer onto the surface.

Cardboard thus obtained can be used in food packaging, in particular itcan be used in pigment coated cardboard, such as fresh fibre cardboard,which has excellent properties of mineral oil migration and fatpenetration prevention.

More specifically, the cardboard according to the present invention ismainly characterized by what is stated in the characterizing part ofclaim 1.

The method according to the present invention is, in turn, characterizedby what is stated in the characterizing part of claim 13.

The use according to the present invention is, in turn, characterized bywhat is stated in claim 27.

The present solution provides considerable advantages. Thus, accordingto the present invention, it is possible to apply online at thecardboard machine a barrier layer onto the surface side of thecardboard, before the pigment coating or before the second or lastpigment coating, in which case excess handling and transport costs areeliminated.

With the barrier solution of the present invention it is possible toreduce the penetration rate of organic solvent vapour through thecardboard to even less than 10%, compared to cardboard without thebarrier.

The barrier has good ability to prevent mineral oil migration and fatpenetration.

Conventional pigment coated fresh fibre cardboard has been observed inlaboratory tests to reduce, by at least 20% of the maximum potential,the total indirect mineral oil migration taking place from the outerpackage through the inner package cardboard. By adding to the cardboardthe barrier solution according to this invention, it is possible toreduce the indirect mineral oil migration even to 1.5% of the maximumpotential.

Since the barrier layers in the barrier solution described in thepresent invention are between the fibre layer and the pigment coatinglayers, that are typically on the surface of the cardboard, and bothouter surfaces have properties similar to the corresponding surface ofconventional cardboard:

-   -   side gluing with dispersion or hotmelt adhesives does not        require unorthodox solutions,    -   printing properties do not differ from conventional, and    -   when reeling at the cardboard machine, blocking of cardboard        does not occur due to the effect of the barrier.

Since the barrier can be applied online at the cardboard machine, thepresent invention enables delivery of the barrier product directly fromthe cardboard mill to the customer, without a separate externalsubcontractor needed to apply the barrier, which will bring significantsavings in logistics costs.

Although, in one preferred embodiment, the barrier is brought to acardboard surface that comprises chemical pulp fibre, it is alsopossible to form the described barrier onto a surface that comprisesmechanical pulp.

The cardboard produced according to the present invention, is entirelypulpable and can be recycled to the cardboard mill process, likeordinary cardboard. In this regard, it clearly differs from, forexample, conventional barrier cardboard brands that comprise extrusionfilms.

In a first embodiment, a barrier dispersion is prepared by dispersing aslate-like kaolin pigment in water and mixing this with either cooked orcold soluble conventional surface-sizing starch slurry.

In another embodiment, the dispersion is applied directly at thecardboard machine.

In a third embodiment, the dispersion is applied directly at thecardboard machine, after surface-sizing and preferably before anypigment coating.

In a fourth embodiment, the dispersion is applied as two separate layersdirectly at the cardboard machine, after the surface-sizing and beforethe pigment coating. The first layer is applied with a film size press,for example, in which case a contour type barrier coating layer isachieved, that follows the surface shapes and the roughness. By applyinga second layer, any pin-holes in the first coating layer are blocked, inwhich case the total thickness of the barrier layer increases and thesurface becomes smooth.

In the following, preferred embodiments are described with reference tothe accompanying photographs.

The electron microscopic picture in FIG. 1A is a side view of a barrierlayer under a pigment coating layer, and

the electron microscopic picture in FIG. 1B shows the same cardboard asin FIG. 1A but without a barrier layer.

In the following description, the terms “spread” and “apply” are usedinterchangeably.

As seen from the above, the present cardboard product is a coated foodcardboard. Such a product comprises a fibre layer (cardboard layer) anda coating layer on at least one side of it.

The fibre matrix of the cardboard may comprise bleached or unbleachedhardwood pulp, bleached or unbleached softwood pulp, bleached orunbleached mechanical pulp, bleached or unbleached chemi-mechanical pulpor recirculated broke pulp used in cardboard production, or mixturesthereof. In particular, mutual mixtures of the above described bleachedand unbleached pulps, respectively, and mechanical or chemi-mechanicalpulps, are possible. Recirculated broke pulp may also be used in any ofthe abovementioned pulps, mechanical pulps and chemi-mechanical pulps.

An example of a multilayer cardboard is a product that comprises incombination

-   -   a first fibre layer having an outer surface and an inner        surface,    -   a second fibre layer, that is arranged at a distance from the        first fibre layer and having an outer surface and an inner        surface, in which case the inner surface of the second fibre        layer is arranged on the inner side of the first fibre layer,        and    -   a third fibre layer, that is arranged between the first and the        second fibre layer, the fibre layers forming the fibre matrix of        the cardboard, and a tracer containing surface-sizing layer        being arranged on the outer surface of the first fibre layer,        that forms the cardboard back.

In such a product, which in principal corresponds to a conventionalfolding boxboard, at least one of the first and second fibre layerscomprises chemical cellulose pulp.

The first and the second fibre layer may comprise bleached or unbleachedsoftwood and/or hardwood pulp. A third fibre layer comprises mechanicalor chemi-mechanical pulp, unbleached or bleached softwood or hardwoodpulp or broke pulp that is recirculated in the cardboard production.

Usually, the grammage of the cardboard layer is approximately 40-750g/m², most suitably approximately 90-500 g/m², in particularapproximately 110-400 g/m².

As an example, it may be mentioned that the grammage of a typicalfolding boxboard is approximately 150-350 g/m². Usually, a foldingboxboard comprises 2-10 layers, in which case the grammage of thesurface layer is approximately 40-80 g/m² and of the back 25-50 g/m².

Since the present solution is applied particularly as an onlineapplication, the cardboard layer to be treated is preferably in the formof a cardboard web, and the barrier layer is brought to the cardboardweb coming from the dewatering of the cardboard machine. It is of coursealso possible to apply the barrier layer onto a sheet-like cardboard.

If desired, and as indicated above, the surface of the cardboard can besized in order to modify its surface properties, for example to seal thesurface and to reduce the moisture absorption, before the barrier layeris applied onto the cardboard layer. Such a surface sizing is in oneembodiment combined with online application of the present barriercomposition.

The surface-sizing layer typically comprises a synthetic, water-solublepolymer or a natural polymer or a derivative thereof. Surface-sizingagents may be divided into several groups, in which case the maindivision is between the cationic and anionic surface-sizing agents.

In addition to these, to a certain extent also reactive sizing agents,such as alkyl-ketene dimer (AKD), that are otherwise mainly used inbeater-sizing, are used in the surface-sizing. Also, perfluorinatedsubstances, such perfluorinated phosphate and perfluorinated alkylpolymers may be used.

Cationic surface-sizing agents are cationic starches and starchderivatives, and corresponding carbohydrate based biopolymers. Examplesof synthetic polymers are, for example, styrene/acrylate copolymers(SA), polyvinyl alcohols, polyurethanes and alkylated urethanes.

Anionic surface-sizing agents include anionic starches and starchderivatives, and corresponding carbohydrate based biopolymers, amongothers carboxymethylcellulose and its salts, and alkylcelluloses such asmethyl and ethyl cellulose. Examples of synthetic polymers arestyrene/maleic acid copolymer (SMA), di-isobutylene/maleic acidanhydride, styrene-acrylate copolymers, acrylonitrile/acrylatecopolymers and polyurethanes and corresponding latex products thatcomprise the same chemical functionalities.

Many of the above-mentioned are supplied as viscous solutions, that areformed of sodium or ammonium salts of the corresponding polycarboxylicacids.

The percentage of the surface-sizing agent in the solution is generallyapproximately 0.01-25% by weight, typically approximately 1-15% byweight. The amount of the surface-sizing agent (grammage) is generallyapproximately 0.1-10 g/m², particularly approximately 0.2-5 g/m², forexample approximately 0.3-3g/m².

The barrier layer is formed onto the cardboard layer, either directly orafter application of the surface-sizing agent. This layer comprises abinder and a slate-like mineral pigment. Most suitably, the barrierlayer is formed with a dispersion coating.

Preferably, the pigment together with the binder, form a networkstructure that is capable of slowing down the penetration of moleculesof unwanted substances, through the barrier layer.

In one preferred embodiment, the barrier layer comprises non-hydrophobicbinder. Examples of suitable binders include starch, such as cooked orcold soluble surface sizing starch slurry, polyvinyl alcohol, ethylenevinyl alcohol, and mixtures thereof. It has been found that thenon-hydrophobic binder is particularly advantageous because it allowsapplying of coating paste onto the barrier layer, in such a way that thecoating layer adheres to and remains on the surface of the barrierlayer.

In one embodiment, the barrier layer comprises of its dry weight 5-70%mineral pigment and 95-30% binder, in particular it comprises of its dryweight approximately 15-60% mineral pigment and 85-40% binder, possiblytogether with dispersion additives.

As examples of the slate-like mineral pigments of the barrier layer maybe mentioned kaolin, talc and mixtures thereof.

In one embodiment, the barrier layer is formed of a dispersion that isprepared by dispersing the slate-like pigment in water and mixing thiswith a non-hydrophobic binder.

According to a more preferred embodiment, the barrier layer is formed ofa dispersion, that is prepared by dispersing a slate-like pigment inwater, in particular in order to generate a pigment dispersion that hasa solids content of over 50%. The dispersion thus obtained is preferablycombined with the binder by mixing, in which case the pigment percentageof the dry matter of the dispersion that is used to form the barrierlayer, is approximately 5-70%, in particular approximately 15-60%.

In one preferred embodiment, the dispersion is applied at the cardboardmachine, i.e. as an online application, after possible surface-sizingand before the pigment coating.

This embodiment will provide interesting effects. Thus, the surface sizewill close the fibrous surface of the web thereby preventing excessivepenetration of the binder of the barrier composition into the web.Instead, the binder remains in the barrier layer. Further, the barrierlayer forms a smooth layer after a preceding surface sizing step, andthis will further reduce the risk of formation of pin-holes. A smoothbarrier layer, obtained as discussed, will also form a smooth base layerfor one or several pigment coating layers. In combination, the pigmentcoating layers will be smooth and they will further improve the barriereffect of the barrier layer, as well as provide for a good printingsurface. Good haptic and visual properties will also be attained.

The dispersion may be applied as a single application, but it can alsobe applied in several layers on top of each other. In a preferredembodiment, the dispersion is applied as at least two separate layers.

In one embodiment, the first dispersion layer is applied with a filmsize press, in which case a contour type barrier layer is achieved, thatfollows the surface shapes and the roughness.

A second layer is applied onto the first layer. It can be applied, forexample, by using a blade or rod, and the application can therefore becarried out, for example, with a blade coating unit or a rod coatingunit.

After the application of the second dispersion layer, or otherdispersion layers, in one alternative, the barrier layers are leftundoctored.

In a preferred embodiment, after the application of the seconddispersion layer, or other dispersion layers, the layers are doctoredwith a rotating or static, smooth or grooved rod, an air knife, or ametallic or ceramic blade. It is also possible to proceed in this way,in case the dispersion coating is brought onto the surface of thecardboard by single application.

Preferably, the dispersion layer is applied from a pigment slurry havinga relatively high dry matter content. In one embodiment, the dry mattercontent of the pigment is 10-40%, in particular 12-39.5%, calculated onthe weight of the slurry.

A typical grammage of the barrier layer is approximately 1-50 g/m²,particularly approximately 5-40 g/m², most suitably approximately 10-30g/m², per side or page of the cardboard.

The barrier layer increases the weight of the product only to a smallextent, typically by approximately 5-30%, compared to a product withouta barrier layer.

In a dispersion that is suitable for forming a barrier layer, it ispossible to use conventional additives and auxiliary agents, such asdispersing agents (for example sodium salt of polyacrylic acid),substances that affect the viscosity of the mixture (for example CMC,hydroxyethyl cellulose, polyacrylates, alginates, benzoate), so-calledslip agents, hardeners used for improving water repulsion, opticalauxiliary agents, anti-foaming agents, pH adjusting agents andpreservatives, in the coating composition. Examples of the slip agentsinclude sulphonate oils, esters, amines, calcium and ammonium stearates;the agents that improve water repulsion include glyoxal; the opticalauxiliary agents include derivatives of diamino stilbene sulphonic acid;the anti-foaming agents include phosphate esters, silicones, alcohols,ethers, vegetable oils; the pH controlling agents include sodiumhydroxide, ammonia; and finally the preservatives include formaldehyde,phenol and quaternary ammonium salts.

After the application of the barrier layer, the cardboard web or sheetto be coated is brought to the front coater.

Based on the above, also the following applications can be mentioned:

In one preferred embodiment, the binder used is cationic starch. Thecationic degree of this may be in the range of DS 0.005 to 0.05.

The proportion of starch in the barrier layer may be 40-85% by weight.

The surface-sizing starch used can be for example corn, potato, tapiocaor pea starch.

In another preferred embodiment, that can be combined with the former, athickening agent is used in the dispersion. The thickening agent usedmay be a synthetic thickening agent, the quantity of which may be even10% by weight of the dry matter. The synthetic thickening agent may be,for example, PVA or polyacrylate. The thickening agent may also be, forexample, carboxymethylcellulose or microfibrillated pulp or a mixturethereof.

In a third preferred embodiment that can be combined with theapplications mentioned above, a hardener is used in the dispersion, inan amount of up to 1% by weight of the dry matter. The hardener used canbe, for example, ammonium zirconium carbonate or potassium zirconiumcarbonate or the like.

In all of the above mentioned application cases, the dry matter may bewithin the range of 5-50%. The Brookfield viscosity can typically bewithin the range of 30-2000 mPas, for example 30-800 mPas.

After that, the coating layer is applied onto the barrier layer.Typically, it comprises binder and light-refracting pigment or a mixtureof pigments. The barrier layer can be dried before application of thecoating layer, but it is also possible to bring the coating layer on thetop of a moist barrier layer.

More preferably, the coating layer comprises pigment that refracts lightbetter than the slate-like mineral pigment of the barrier layer. Thecoating layer may be generated by using one fine mineral pigment, or amixture of several fine pigments, as follows:

Coating pigment I 10-90 parts by weight (for example, fine carbonate)Coating pigment II 10 to 90 parts by weight (for example, fine kaolin)Pigments in total 100 parts by weight Binder 1-20 parts by weightAdditives and auxiliary agents 0.1-10 parts by weight Water balance

Water is added to the coating paste of this type in such a way that thedry matter content is typically 50-75%.

As examples of suitable pigments may be mentioned: precipitated calciumcarbonate, ground calcium carbonate, calcium sulphate, calcium oxalate,aluminium silicate, kaolin (anhydrous aluminium silicate), aluminiumhydroxide, magnesium silicate, talc (hydrous magnesium silicate),titanium dioxide and barium sulphate, and mixtures thereof. Syntheticpigments can also be used. Of the pigments mentioned above, the mainpigments are kaolin, calcium carbonate, precipitated calcium carbonateand gypsum that in general constitute over 50% of the dry solids in thecoating composition. Calcined kaolin, sodium dioxide, satin white,aluminium hydroxide, sodium aluminate and plastics pigments areadditional pigments, and their amounts are in general less than 25% ofthe dry solids in the mix. Of the special pigments, special-qualitykaolins and calcium carbonates, as well as barium sulphate and zincoxide, should be mentioned.

In particular, the pigment of the coating layer is calcium carbonate,precipitated calcium carbonate, titanium dioxide, calcium sulphate,aluminium silicate, aluminium hydroxide, magnesium silicate or bariumsulphate, or a mixture thereof.

In a refractive pigment-containing coating layer, it is possible to useany known binder that is commonly used as a binder in coating colours inthe manufacturing of paper. Besides individual binders, it is possibleto use mixtures of binders. Examples of typical binders includesynthetic latexes made of polymers of ethylenically unsaturatedcompounds, for example butadiene-styrene type copolymers that possiblyalso comprise a comonomer that comprises a carboxyl group, such asacrylic acid, itaconic acid or maleic acid, and polyvinyl acetate havingcomonomers that comprise carboxyl groups. Together with theabovementioned agents, it is also possible to use as binders, forexample, water-soluble polymers, starch, CMC, hydroxyethyl cellulose andpolyvinyl alcohol.

In the coating composition, it is possible to use conventional additivesand auxiliary agents (similarly as in the above barrier dispersion).Examples include dispersing agents, agents that affect the viscosity ofthe mixture, slip agents, hardeners, optical auxiliary agents,anti-foaming agents, pH controlling agents, and preservatives.

In one embodiment, the cardboard layer is coated only on one side, andthere is a barrier layer only between this layer and the cardboardlayer.

In another embodiment, on both sides of the cardboard layer there arecoating layers, and preferably, barrier layers comprising binder andslate-like mineral pigment, are arranged between both coating layers andthe corresponding cardboard surfaces.

The grammage of the coating layer can vary within wide limits, forexample, according to the number of layers of the coating, and istypically approximately 0.5-50 g/m^(2/)side, usually approximately 5-40g/m²/side, for example approximately 5-30 g/m²/side. The amount ofcoating may be different in the surface and the back layers. Forexample, in the surface that forms the outer surface of the cardboard,the amount of coating is often approximately 10-35 g/m², and in theback, i.e. inner surface, approximately 5-20 g/m².

As described above, in laboratory, pilot and production scaleexperiments, the barrier according to the present invention was observedto have a good ability to prevent mineral oil migration and fatpenetration.

Due to its network structure starch, which acts as the binder, slowsdown and prevents the penetration of unwanted substance molecules, ingas or liquid phase, through the barrier layer. The slate-like pigmentcreates in the barrier layer a complex structure that is filled withbinder, in which case the path of the unwanted molecules is extendedmany times, compared to the binder layer only. The pigment also acts asa filler which reduces the production costs of the barrier.

When the barrier layers are located under the pigment coating, insteadof the cardboard surfaces, the appearance, gluing, printing and otherprocessing properties of the cardboard are maintained as unchanged aspossible, compared to a standard product.

It should also be noted that there may be several coating layers in thecardboard. In one embodiment, the barrier layer is placed between thefibre layer which possibly is surface-sized and the coating layer.However, it is also possible to place the barrier layer between thelayers of a multilayer coating.

The coating layers are primarily those that comprise light-refractingpigment and provide the product with the required opticalcharacteristics, for example brightness and opacity. ISO brightness is,for example over 85, in particular over 87, most suitably over 90, andthe opacity is better than 85, in particular better than 87%.

The following non-restricting examples illustrate the preferredembodiments.

The cardboards used in the examples are folding boxboards, in which casethe middle layer of the multi-layer cardboard is formed of mechanicalpulp, and its surface layers of chemical pulp, i.e. cellulose.

EXAMPLE 1 Barrier Coating of Cardboard A in the Laboratory

Slate-like kaolin was elutriated in a laboratory mixer by firstmeasuring the desired amount of water into a mixing vessel and thenadding the dispersant while stirring. After that, dry kaolin powder wasadded gradually and the stirring was continued until the slurry becameuniform. The stirring was continued for an additional hour. The drymatter content of the kaolin slurry was 67%.

A cooked surface sizing starch, which acts as the binder, was preparedin a production scale apparatus by first elutriating the starch in waterin a suspender and then cooking it in a continuous digester, anddiluting it to a desired dry matter content (14.5%).

The kaolin slurry and the cooked starch were mixed in such a way thatthe mixture ratio of their mass fractions was 20/80, in the ordermentioned, and the dry matter content of the mixture was 17%.

The prepared mixture was used to coat cardboard A in the laboratory. Thecardboard coating was carried out twice with a grooved bar number 2(ERICHSEN GmbH & Co. KG K 101 Bar No. 2=12 μm (red)) at a rate of 4m/min. The coating was dried after each application with an infrareddryer for a period of 1 minute.

The penetration rate of the organic solvent vapour was measured from thebarrier coated cardboard. The measurement was carried out by weighing aconstant amount of n-heptane in a vessel, on which was placed tightlythe board to be examined, with a known surface area. Evaporation of theheptane through the cardboard was monitored by weighing, in which casethe following penetration rates of n-heptane were achieved.

TABLE Base cardboard 1.25 g/dm²/day Cardboard coated with a 20/80mixture of 0.13 g/dm²/day kaolin and surface sizing starch

Example 1 shows the performance of a barrier layer, according to thepresent invention, in preventing penetration of the organic solventvapours.

EXAMPLE 2 Barrier Coating of Cardboard B in the Laboratory

A kaolin-surface sizing starch mixture was prepared as in Example 1 insuch a way that the mixture ratio of their mass fractions was 50/50, inthe order mentioned, and the dry matter content was 18%. To the mixturewas added a synthetic thickener in such a way that the Brookfieldviscosity was 200 mPAs. Cardboard B was coated with this mixture twicewith a laboratory coater as in Example 1. The penetration rate of theorganic solvent vapour, which was measured as in Example 1, was 0.10g/dm²/day.

Example 2 shows that with the addition of a thickener it is possible tofurther improve the barrier properties.

EXAMPLE 3

Barrier Coating of Cardboard C with a Pilot Coating Machine

The back of a normal pigment coated cardboard was coated twice, with amixture of kaolin-starch, according to Example 1, in such a way that themixture ratio of their mass fractions was 50/50, in the order mentioned,and the dry matter content was 18%. A thickener and hardener were addedinto the barrier mixture until a good runnability was achieved at thecoating station, and a uniform barrier layer, both of which wereobserved visually. The grammage of both of the barrier layers was 6g/m².

Indirect migration from the mineral oil containing outer packagingmaterial was, without maximum potential. The indirect migration wasdetermined by the method according to the following standard: EN 14338Paper and board intended to come into contact with foodstuffs—Conditionsfor determination of migration from paper and board using modifiedpolyphenylene oxide (MPPO) as a stimulant.

Example 3 shows that when there is an additional conventional pigmentcoating layer, the barrier properties are further improved.

EXAMPLE 4

Manufacturing of Light Online Barrier Cardboard with a Pilot CoatingMachine

The back of an uncoated base cardboard was surface sized and the surfaceside of the cardboard was coated with a film size press, with a mixtureof kaolin and cooked starch, according to Example 1, the mixture ratioof the mass fractions of which was 50/50. After that, the surface sidewas coated for a second time by using a blade coater, with a mixture ofkaolin and cold soluble starch, the mixture ratio of the mass fractionsof which was 50/50. Finally, a pigment coating was applied onto thesurface side, as well as onto the reference cardboard, which wasotherwise prepared in the same way, but instead of the two barrierlayers, there was only a surface sizing also on the surface side, underthe pigment coating.

Permeation rates of organic vapours were as follows:

Uncoated cardboard 0.93 g/dm²/day Reference cardboard 0.41 g/dm²/dayLight online barrier cardboard 0.16 g/dm²/day

Indirect migration from the outer packaging material comprising mineraloil was 4.8% of the maximum potential.

Example 4 shows that it is possible to prepare the barrier layer underthe pigment coating.

1. Coated food cardboard which comprises a cardboard layer and a coatinglayer on at least one side of it, further comprising a barrier layerbetween the coating layer and the cardboard containing binder andslate-like mineral pigment.
 2. The cardboard according to claim 1,wherein the barrier layer is formed by dispersion coating.
 3. Thecardboard according to claim 1, wherein the barrier layer comprisesnon-hydrophobic binder.
 4. The cardboard according claim 1, wherein thepigment and the binder together form a network structure, which iscapable of slowing down the penetration of unwanted substance moleculesthrough the barrier layer.
 5. The cardboard according claim 1, whereinthe barrier layer comprises of its dry matter 5-70% mineral pigment and95-30% binder, in particular it comprises of its dry matterapproximately 15-60% mineral pigment and 85-40% binder, possiblytogether with the additives of the dispersion.
 6. The cardboardaccording to claim 1, wherein the binder of the barrier layer is starch,such as cooked or cold soluble surface sizing starch slurry, polyvinylalcohol, ethylene vinyl alcohol or a mixture thereof.
 7. The cardboardaccording to claim 1, wherein the slate-like mineral pigment of thebarrier layer is kaolin, talc or a mixture thereof.
 8. The cardboardaccording to claim 1, wherein the weight of the barrier layer isapproximately 1-50 g/m², in particular approximately 5-40 g/m², mostsuitably approximately 10-30 g/m², per cardboard side.
 9. The cardboardaccording to claim 1, wherein the coating layer comprises binder andlight-refracting pigment or mixture of pigments.
 10. The cardboardaccording to claim 1, wherein the coating layer comprises pigment, whichrefracts light better than the slate-like mineral pigment of the barrierlayer, in particular the pigment of the coating layer is calciumcarbonate, titanium dioxide, calcium sulphate, aluminium silicate,aluminium hydroxide, magnesium silicate or barium sulphate or a mixturethereof.
 11. The cardboard according to claim 1, wherein on both sidesof the cardboard layer there are coating layers and preferably, barrierlayers that comprise binder and slate-like mineral pigment are arrangedbetween both coating layers and the corresponding cardboard surfaces.12. The cardboard according claim 1, wherein there is a surface sizingagent layer between the cardboard surface and the barrier layer.
 13. Amethod of producing coated cardboard, according to which method onto thecardboard is applied a coating mixture that comprises light-refractingpigment, in order to form a coating layer, wherein between the coatinglayer and the cardboard surface is formed, by dispersion coating, abarrier layer that is comprised of binder and slate-like pigments. 14.The method according to claim 13, wherein the barrier layer is formed ofa dispersion that is prepared by dispersing slate-like pigment in waterand mixing this into a non-hydrophobic binder.
 15. The method accordingto claim 13, wherein the barrier layer is formed of a dispersion that isprepared by dispersing slate-like pigment in water, particularly inorder to generate a pigment dispersion that has a solids content of over50%, and this dispersion is combined with a binder by mixing, in whichcase the pigment share of the dry matter of the dispersion that is usedto form the barrier layer, is approximately 5-70%, in particularapproximately 15-60%.
 16. The method according to claim 13, wherein thedispersion is applied at the cardboard machine, after any surface sizingand before any pigment coating, preferably the dispersion is applieddirectly at the cardboard machine after surface sizing and beforepigment coating.
 17. The method according to claim 13, wherein thedispersion is applied as at least two different layers.
 18. The methodaccording to claim 17, wherein the first dispersion layer is appliedwith a film size press, such that a barrier layer is achieved thatfollows the surface shapes and the roughness of the cardboard web. 19.The method according to claim 17, wherein the second dispersion layer isapplied with a blade coating unit or a bar coating unit.
 20. the methodaccording to claim 17, wherein after the application of the seconddispersion layer, the barrier layers are left unformulated or formulatedby using a rotating or static, smooth or grooved bar, an air knife, or ametallic or ceramic blade.
 21. The method according to claim 13, whereinthe dispersion layer is applied from the pigment slurry, the dry mattercontent of which is 10-40% calculated by weight of the slurry.
 22. Themethod according to claim 13, wherein a barrier layer is formed, thegrammage of which is approximately 1-50 g/m², in particularapproximately 5-40 g/m², most suitably approximately 10-30 g/m², percardboard side.
 23. The method according to claim 13, wherein anon-hydrophobic binder, polyvinyl alcohol, ethylene vinyl alcohol or amixture thereof, is used to form the dispersion.
 24. The methodaccording to claim 13, wherein the cardboard is surface sized before itis dispersion coated with a dispersion that forms the barrier layer. 25.The method according to claim 13, wherein the cardboard is coated,possibly after surface sizing, before it is dispersion coated with adispersion that forms the barrier layer.
 26. The method according toclaim 13, wherein a coating layer is formed that comprises pigment thatrefracts light better than the late-like mineral pigment in the barrierlayer, in particular the coating layer pigment is calcium carbonate,precipitated calcium carbonate, titanium dioxide, calcium sulphate,aluminium silicate, aluminium hydroxide, magnesium silicate or a mixturethereof.
 27. The method according to claim 13, wherein on both sides ofthe cardboard layer, are brought coating layers that comprise slate-likemineral pigment, are arranged between both coating layers and thecorresponding cardboard surfaces.
 28. (canceled)
 29. (canceled)